1. Field of The Invention
The present invention relates to a fuel injection control apparatus for an internal combustion engine, which controls an electromagnetic injector (fuel injection valve) to supply fuel into a combustion chamber of the internal combustion engine.
2. Description of The Prior Art
A highly-pressurized fuel injection apparatus, equipped with e.g. common rail type injectors, has been conventionally used as an apparatus supplying fuel into combustion chambers of a diesel engine. This type apparatus normally uses an injector having an electromagnetic valve to atomize fuel and supply it into the combustion chamber. In this case, an injection rate is univocally determined in accordance with a diameter of an injection nozzle of the injector and an injection pressure of the same.
In general, diesel engines have a disadvantage in generating a significant amount of combustion noise and NOx due to delay in firing fuel in the combustion chamber. This kind of noise or NOx problem cannot be solved by the injector itself, in the case where this injector has an injection rate being determined in the univocal manner as described above.
A known countermeasure solving this problem is a so-called pilot injection technology (For example, refer to Japanese Unexamined Patent Application Nos. SHO 63-5140 and 62-129540), in which a small amount of fuel is injected into the combustion chamber prior to an ordinary (main) fuel injection. According to this technology, the above problem can be solved by actuating the injector twice to inject fuel separately during one compression stroke of the diesel engine. However, this technology accompanies other problems as follows.
As the injector has a coil with a significant amount of inductance, an exciting current supplied to the injector causes a delay in its building-up stage due to this coil inductance. Namely, the injector causes a delay in the initiating stage of its fuel injection period. In order to solve this inherent delay accompanying the injector, there is known a technology disclosed, for example, in Japanese Patent No. SHO 49-45248.
In this conventional technology, the injector is supplied with two kinds of currents as shown in FIG. 20(a). One is a valve-opening current having a predetermined height and length corresponding to an injection pulse for an ordinary fuel injection. The other is an exciting current having a relatively high but short pulse waveform corresponding to a trigger pulse, which is given from a high-voltage generator.
This arrangement can improve the injector to promptly initiate fuel injection without causing an adverse delay.
However, if this technology is incorporated with the fuel injection system performing the pilot injection, an overall circuit configuration must become large in size. Because it requires a doubled-size circuit, comprising two independent current supply circuits for quickly discharging electrostatic energy charged in the capacitor, for supplying first and second, i.e. pilot and main, exciting currents corresponding to two injections as shown in FIG. 20(b).
Furthermore, as the injector has a solenoid to drive the electromagnetic valve, a residual magnetic flux remains in this solenoid due to an exciting current having flowed through the solenoid in response to the first activation of the injector (i.e. a pilot injection). Furthermore, a significant amount of pressure pulsation remains in the injector or its associated pressure pipe due to this pilot fuel injection.
The residual magnetic flux and pressure pulsation are not preferable for the precise injector control because they give adverse effect to the response speed of the injector. In this case the response speed becomes very fast; therefore, the fuel injection initiates earlier than an expected (commanded) timing, accompanied with an unanticipated increase in fuel injection amount due to this earlier valve opening.
In more detail, the residual magnetic flux is enlarged as a pilot injection period TP increases and an injection interrupt period TI decreases as shown in FIG. 2, which illustratively shows the relationship among the pilot injection period TP, the injection interrupt period TI, and the residual magnetic flux density.
Accordingly, a valve-opening time, i.e. a time lag between the current supply and an actual opening of the valve, becomes short with increasing residual magnetic flux density as shown in FIG. 3, which illustratively shows the relationship between the valve-opening time of the injector and the residual magnetic flux density. This gives adverse effect to the injection initiating timing and fuel injection amount in the second (main) activation of the injector executed after the first (pilot) injection.
As already explained in the foregoing description, providing a circuit for supplying a trigger pulse current in each activation of the injector is one of countermeasure for solving this problem but will not be preferable because of large size and complicatedness in its circuit configuration.
A means for supplying a preliminary current, as shown in Japanese Unexamined Patent Application No. SHO 62-276242, is also effective to solve this problem but will encounter with difficulties in ensuring valve closing and suppressing electric power consumption.